In an apparatus of the above-described type the filter beds must periodically be freed from dust which has deposited therein. This is usually effected by back-purging with a purging gas stream while the unit is shut off for the normal flow of gas therethrough. The duration and frequency of the purging phases will depend on the requirements in each case.
Fixed intervals of times may be programmed or the purging phases may be initiated when predetermined measured values have been reached.
In cases in which the raw gas does not exhibit appreciable fluctuations in rate, temperature and dust loading, a simple timer will be used. More differentiated control means will be required under strongly changing conditions.
A control which is dependent on the dust content of the pure gas is only of restricted utility because the measurements are relatively expensive and closed-loop control systems depending on said measurements is too slow in most cases. For this reason, it is usual to measure the pressure drop across each filter bed. That pressure drop increases continuously with the dust loading and when it exceeds a predetermined value a control pulse for initiating the automatic purging phase can be generated.
However, it has been found that satisfactory results cannot be obtained in such cases if the conventional granular bed filter is preceded by a centrifugal separator because the dust loading in the filter bed often has a highly irregular distribution so that certain portions of the filter bed are utilized only to a subaverage degree and other portions are overloaded with dust between two purging phases.
These phenomenon result in erroneous measurements and will involve a need for relatively long purging times if the portions which have been overloaded with dust are also to be cleaned as completely as could be effected by the existing back-purging system in case of a uniform dust loading. Longer purging times will result in a less favorable ratio of the deducting time to the purging time and--if a plurality of apparatuses are used in the usual manner--fewer apparatus will be available at the same time for a collection of dust and more units will have to be provided for a given application.
It is apparent that an irregular loading of the filter beds will have an adverse effect not only on the accuracy of measurement but also on the operating costs (higher purging air requirement) and on the plant costs (more units).